Large pore zeolites with high silica to alumina ratios, i.e., of at least four, are desirable because of their particular catalytic selectivity and their thermal stability; the latter is a property particularly important when the zeolite is used as catalyst or in adsorption procedures wherein exposure to high temperatures would be expected. Although faujasite zeolites having silica to alumina ratios of less than four can be readily synthesized by a variety of methods, as disclosed, e.g., in U.S. Pat. Nos. 2,882,244 and 4,178,352, methods for preparing faujasite polymorphs of higher ratios generally involve several weeks of crystallization and result in poor yields of product, as reported by Kacirek, J. Phy. Chem., 79, 1589 (1975). The process of this invention is specifically directed to making products identified as comprising faujasite (FAU), "Breck-6" (BSS or EMT) and intergrowths of the two (i.e. the faujasite family of zeolites).
The use of quaternary ammonium salts as templates or reaction modifiers in the preparation of synthetic crystalline aluminosilicates (zeolites), first discovered by R. M. Barrer in 1961, has led to preparation of zeolites with high silica to alumina ratios which are not found in nature. A review provided by Barrer in Zeolites, Vol. I, p. 136 (October, 1981) shows the zeolite types which are obtained using various ammonium organic bases as cation. In addition, Breck, Zeolite Molecular Sieves, John Wiley (New York, 1974), pp. 348-378, provides a basic review of zeolites obtained using such ammonium cations in the synthesis thereof, as does a review by Lok et al. (Zeolites, 3, p. 282, 1983)). The faujasite family of materials is well reviewed by Treacy et al. (Proc. Royal Soc., A, 1996) and in U.S. Pat. No. 5,116,590, included herein by reference.
Alkylammonium templates are the high cost component of the synthesis and therefore may be a dominant component of the manufacturing economies. Many attempts to recycle the "template" containing mother liquor (ML) from succeeding zeolite preparations fail because in addition to the "template" the ML contains polymerized reactants and impurity nuclei. In the case of high silica faujasite type preparations (FAU/BSS) ZSM-5 and gismondine are the major problem impurities. Although such methods as "template" recrystallization, steam stripping of ML, ion-exchange and centrifugation may be used to recover "template", all require expensive equipment and are energy intensive. The subject processes do not require such expenditures.
We have discovered that by using controlled composition, controlled release, reagents, impurity growth can be controlled and the raw, filtered, ML can be used to make high purity high silica products. By using silica and alumina reagents in a combined form (i.e. one example is silica-alumina gel, similar to the old FCC amorphous catalyst) we can use recycled template liquors in the synthesis, whereas recycle ML cannot be used in a repeat of the standard synthesis using conventional dissolved silicates and aluminates. The use of such combined silica-alumina raw material also provides a new method for making the base case ECR-32, allowing a continuous process of synthesis using one raw material and continuous recycle of all waste liquids. Using such methods the process economics are improved in a major way in an more environmentally friendly process, further reducing effluent waste treatment costs. It is recognized that other "T-atom" forms (e.g. Ca, Zn, Fe, B, Ti, V alone or in combination with Al) may be made by appropriate substitutions in the gel.
Appropriate templates for making the subject materials include tetra-alkyl ammonium ions having methyl, ethyl, propyl or butyl substituents or their hydroxyl substituted forms (U.S. Pat. Nos. 4,714,601; 4,879,103; 4,931,267; 4,965,059; 5,116,590), crown ethers (Zeolites, 10, p. 546 (1990)) alkaline polyoxides (U.S. Pat. No. 5,192,520), and others well known in the art.
The objective of the present invention is to develop improved zeolite preparation methods yielding high silica materials wherein a significant portion of the organic templates are derived from templates recycled from earlier preparations. The target zeolite group are the faujasite polymorphs, particularly those designated ECR-32/ECR-4 (see U.S. Pat. No. 4,714,601, U.S. Pat. No. 4,965,059, and U.S. Pat. No. 4,931,267), ECR-30 (U.S. Pat. No. 4,879,103) and ECR-35, (U.S. Pat. No. 5,116,590) and like materials. The principal objective of this invention is to significantly improve the process economics for making such materials, and improving the process effluent and waste properties, thus reducing pollution control costs.